1. Field of the Invention
The present invention relates to certain novel hydrazides and the use thereof as curing agents for epoxy resins.
2. Description of the Prior Art
Epoxy resins are widely employed as electric insulating materials, various moulded products, adhesives or coatings because they give valuable cured resins having excellent mechanical, electrical and chemical properties when cured with suitable curing agents, for example acid anhydride and amine curing agents. However, epoxy resin composition incorporating amine curing agents are cured rapidly at ordinary temperature and at elevated temperature, and hence they lack storage stability. Also, epoxy resin compositions incorporating acid anhydride curing agents are stable at ordinary temperature, but heating for a long period of time at elevated temperature is required for full curing. Usually, tertiary amines, quaternary ammonium compounds or organo metal complexes are further added to the composition for the purpose of accelerating curing rate. However, the addition of such cure accelerator impairs storage stability markedly.
So-called latent curing agents which are compatible with epoxy resins to form a composition which is stable at relatively low temperature and which is rapidly cured when heated to elevated temperature are eagerly desired. In the field of coating, particularly, curing agents are desired which give colorless and transparent cured epoxy resin, from the view of tone of color. Representative compounds which have been heretofore proposed as latent curing agents are dicyandiamide, dibasic acid hydrazide, boron trifluoride-amine adduct, guanamine and melamine. Among these compounds, dicyandiamide, dibasic acid hydrazide and quanamine are useful in formulating epoxy resin compositions having excellent storage stability, but full curing by means of these compounds could be achieved only by heating at higher temperature than 150.degree. C. for a long time. Also, boron trifluoride-amine adduct is hard to treat owing to its high hygroscopic property, and it affects the physical properties of the cured resin adversely.
There has been heretofore known almost no latent epoxy curing agent which causes rapid curing at moderate elevated temperature (that is 100.degree. C.-150.degree. C.) and which gives an epoxy resin composition having excellent storage stability at ordinary temperature.